Changes in Phenolic Compounds During Plum ... - ACS Publications

Chapter 22

Changes in Phenolic Compounds During Plum Processing 1

2

Hui-Yin Fu , Tzou-Chi Huang , and Chi-Tang Ho

1

1

Department of Food Science, Cook College, Rutgers, The State University of New Jersey, New Brunswick, NJ 08903 Department of Food Science and Technology, National Pingtung Polytechnic Institute, Pingtung, Taiwan

Downloaded by UNIV OF ROCHESTER on June 23, 2017 | http://pubs.acs.org Publication Date: October 1, 1992 | doi: 10.1021/bk-1992-0506.ch022

2

Semi-dried pickled plums are the most important processed fruit product in Taiwan. The formation of the red-brown color on the skin of the product can only be achieved through a full sun-drying process. This variation in pigment is the most noticeable change that occurs during the processing. The total quantity of polyphenols decreased significantly in the sun-dried sample. A mixture of leucoanthocyanin and (+)-catechin purified from plum flesh by preparative TLC was used as a model system to study the mechanism for the formation of the pigment. A red-brown pigment similar to that in the sun-dried plums was isolated and purified after exposure to tungsten light. Radiation in the visible portion of sunlight appeared to catalyze the photopolymerization of the red-brown pigment.

Plums (Plunus mume Sieb. et Zucc.) have been cultivated in China for at least 3000 years. They were introduced to Taiwan and are now common in the mountainous areas. Plum fruits are harvested in the late spring, and are processed into different commercial products. Among them, semi-dried plums are the most popular. The annual production is estimated at 5,000,000 tons and is exported exclusively to Japan. High salt (ca. 25%) pretreatment for 3 to 6 months was performed traditionally to obtain the characteristic aroma. The pink color on the skin of the product developed as a result of exposure to sunshine for a minimal period of two days (7 hours exposure per day). The water content was reduced from 95% in plum flesh to about 60% in the final product. This paper reports the possible mechanism for the formation of the desirable red-brown colors in sun-dried pickled plums. 0O97-6156/92/0506-O287$06.00/0 © 1992 American Chemical Society

Ho et al.; Phenolic Compounds in Food and Their Effects on Health I ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

288

PHENOLIC COMPOUNDS IN FOOD AND THEIR EFFECTS ON HEALTH I


Phenolic Compounds in Plums An HPLC equipped with a Photodiode Array UV-VIS detector was used to characterize phenolic compounds in plums. Identification was accomplished by comparing their retention times and UV absorption spectra with those of authentic standards. Phenolic compounds extracted from plums by ethyl acetate were separated into acidic and neutral fractions using a Sep-Pak C18 cartridge according to the method of Lee and Jaworski (1). The acidic fraction contained mainly three isomers of caffeoylquinic acids (structures shown in Figure 1): 3-caffeoylquinic acid (neochlorogenic acid) (2.7 mg/g dry matter), 4-caffeoylquinic acid (cryptochlorogenic acid) (1.9 mg/g dry matter) and 5-caffeoylquinic acid (chlorogenic acid) (1.9 mg/g dry matter) as shown in Table I. The high concentration of caffeoylquinic acids in plum flesh is in accord with the results reported for other fruits of the same genus, prunus

(2-3).

The neutral fraction contained mainly (-)-epicatechin (0.53 mg/g dry matter) and (+)-catechin (0.4 mg/g dry matter), at about the same level as other fruits of the same genus. Table I. Compound Acidic Fraction 3-caffeoylquinic acid 4-caffeoylquinic acid 5-caffeoylquinic acid Neutral Fraction (+)-catechin (-)-epicatechin procyanidins B-l B-2 B-3 B-4

Phenolic Compounds in Plum Flesh Concentration (mq/q dry matter) 2.7 1.9 1.9 0.45 0.53 0.06 0.09 0.08 0.05

Changes of Phenolic Compounds during Sun-Drying Processing During the processing of semi-dried pickled plums, significant color changes were observed. The original green color of plum flesh changed to a yellowish color after curing, to a light reddish color during the early stages of the sun-drying, and to a red-brown color after sun-drying. Interestingly, the red-brown color on the skin of the product could only be formed through the full sun-drying process. For oven-dried products, a black-brown instead of a red-brown color developed on the skin. According to data previously reported by us (4) it was found that the content of both total chlorophyll and carotenoids decreased gradually through the whole process. Total chlorophyll wa^s reduced from 14.0 mg/g dry matter in the flesh to 1.04 mg/g dry matter after being kept in brine for 4 weeks, and dropped to a negligible amount at the end of the two-day sun-drying period. A similar trend was observed for the total carotenoids. They decreased from 2.64 mg/g 9



22. FU ET AL.


dry matter in the flesh plums to 1.05 mg/g dry matter after being kept in brine, and to 0.34 mg/dry matter after being sun-dried. Quantitative changes in the major phenolic compounds at different stages of the processing of semi-dried pickled plum are shown in Figures 2 and 3. HPLC was used to monitor the degradation of phenolic compounds. Among the neutral phenolics in plums (-)-epicatechin degraded most drastically, followed by (+)-catechin. It was estimated that 47% of (-)-epicatechin and 78% of (+)-catechin remained in the cured sample, and only 1.2% and 15.6%, respectively, were left in the sun-dried sample. It was noticeable that (-)-epicatechin degraded almost completely during the curing and sun-drying processes. Decrease of (-)-epicatechin in curing may be due to the high specific activity of polyphenol oxidase for this phenolic as compared with that of (+)-catechin (4). For acidic phenolics, there were 44% of 3-caffeoylquinic acid, 37% of 4-caffeoylquinic acid and 35% of 5-caffeoylquinic acid remaining in cured plums, and only 15%, 16% and 10%, respectively, left in the sun-dried plums. All three isomers of caffeoylquinic acid degraded at similar rates. This probably indicated that the oxidation of o-dihydroxy groups of the caffeoylquinic acids was the main mechanism for their degradation. Properties of Red-Brown Pigment The formation of a red-brown pigment was the most noticeable change occurring during the processing of sun-dried pickled plums. The red-brown pigments were first extracted with cold acetone from the fully sun-dried plums. Addition of one and a half volumes of chloroform to the aqueous filtrate resulted in a biphasic separation. The aqueous phase was concentrated and then subjected to g e l - f i l t r a tion on a Sephadex G-25 column according to the method of Somers (5). Two main fractions were obtained which, from their absorption at 420 nm, indicated separation between polymeric proanthocyanin (Fraction I; elution volume 10-40 mL) and oligomeric flavan (Fraction II; elution volume 40-100 mL) as shown in Figure 4. Fraction I was a polymeric polyphenol with a minimum molecular weight of 2,000 estimated for complete exclusion from this particular gel. This fraction was immobilized on a cellulose TLC plate and developed in a butanol-acetic acid-water (BAW; 4:1:1) mobil phase. A tan-colored band at the origin was extracted with hot water, and UV absorption maximum at 280 nm was read after suitable dilution in the same solvent (Figure 5). In the treatment with butanol-hydrochloric acid (100:5) at 100°C, Fraction I gave mainly cyanindin. Cyanidin was liberated and characterized by HPLC. The color of Fraction II was yellow-brown. HPLC analysis showed that the second fraction was a mixture of oxidation products derived from plum polyphenols. At least 5 peaks were observed in the HPLC profile. The UV absorption spectrum of one of the major peaks was very similar to that of the caffeoylquinic acid oxidation product (6). Both compounds showed a maximum absorbance near 320 nm.



3- C a f f e o y l q u i n i c acid (NeochTorogenic R-j= R ; R - R^ - H

acid):


4

4- Caffeoylqui n i c acid (Cryptochlorogenic a c i d ) : = R^ Rj R-j R^ H =

=

5- C a f f e o y l q u i n i c acid (Chlorogenic a c i d ) : Rç Rj R R^ H =

Figure 1.

3

=

=

Structures of caffeoylquinic acids.

TIME ( days )

ure 2

Evolution of neutral phenolics in plum during sun-drying.



Figure 3.

Evolution of acidic phenolics in plum during sun-drying.


H

4

s©

I

s

a.

to c

s

!

?

S'

ι

>

W H

•π cl


292


EFFLUENT VOLUME (mL)

Figure 4. Gel filtration of red-brown pigments from sun-dried plum on Sephadex G-25.

WAVELENGTH (nm)

Figure 5. UV absorption spectra of red-brown pigments extracted from sun-dried plum.


22. FU ET AL.


Formation of Red-Brown Pigment


A mixture of (+)-catechin and procyanidins (20:1, by weight) purified from plum flesh by preparative cellulose TLC was set up as a liquid model system to study the formation mechanism of the pigment (J). A red-brown pigment was isolated and purified after exposure to radiation from a tungsten lamp at 40°C. The sample exposed to sunlight produced a similar pigment. Both absorption spectrum of the main fraction, and the effluent volume of the red-brown pigment agreed well with that found in the sun-dried plums as shown in Figure 5. A model system composed of (-)-epicatechin was set up to confirm the occurrence of photopolymerization during the sun-drying process. The rate of red-brown pigment formation was found to be directly related to the reaction temperature. Absorbance at 420 nm increased as the heating temperature increased. The browning reaction of (-)-epicatechin was also found to be pH-dependent. A minimum pH value of 5 at 45°C is required for the red-brown pigment formation as shown in Figure 6. No significant browning reaction was observed below pH 4 after heating for 6 hours. Addition of procyanodin to the (-)-epicatechin solution increased the rate of the browning (Figure 7). A photo-sensitive constituent, riboflavin, doubled the rate of the browning reaction, and pheophytin further enhanced the reaction. The red-brown pigment formation was monitored by Sephadex G-25 column chromatography. The elution patterns of (-)-epicatechin solution samples heated for 2.4 and 6 hours, respectively, are shown in Figure 8. A molecular weight of about 2000, and a UV absorption maximum at 280 nm were characterized for the first fraction. Cellulose powder coated with (+)-catechin and leucoanthocyanin was used as the solid model system. A red-brown pigment was produced in the samples exposed to radiation from a tungsten lamp as well as the sample exposed to sunlight. It was concluded that (+)-catechin, (-)-epicatechin and procyanidins are the major constituents that participate in the formation of the red-brown pigment. Radiation in the visible portion of the spectrum appears to catalyze the photopolymerization of the redbrown pigment. The red-brown pigments, the flavalans, may be formed by linkages between C-4 of one (+)-catechin or (-)-epicatechin, and C-8 of another unit through the oxidative or photosensitive polymerization reaction postulated by Creasy and Swain (8). Leucocyanidin may also condense with (+)-catechin or (-)-epicatechin to generate procyanidins, B1-B4 and, consequently, to the polymeric redbrown pigment following the mechanism analogous to that proposed by Delcour et a l . (9). Cell destruction may take place during the sundrying process. Overnight tempering treatment (plums in crates covered with plastic film) enhances the diffusion of plasmic polyphenolic compounds. Accumulation of these phenolic acids on the epidermis cell leads to the pronounced oxidation reaction of odiphenols.



294


Time (hr)

Figure 6. Effects of pH values on the browning reaction in (-)epicatechin model system.

Figure 7. Effects of photosensitizers on the browning reaction in (-)-epicatechin model system.



22. FU ET AL.


40

80

120

EFFLUENT VOLUME (mL)

Figure 8.^ Sephadex G-25 gel-filtration curve of red-brown pigments from heated (+)-catechin model system.

Acknowledgments This publication, New Jersey Agricultural Experiment Station No. D10205-2-92, has been supported by State Funds and Hatch Regional Project NE-116. We thank Mrs. Joan B. Shumsky for her secretarial aid. Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9.

Lee, C. Y.; Jaworski, A. W. Amer. J. Enol. Vitic. 1987, 38, 277281. Raynal, J.; Moutounet, M.; Souguet, J.-M. J. Agric. Food Chem. 1989, 37, 1046-1050. Herrmann, K. CRC Crit. Rev. Food Sci. Nutr. 1989, 28, 315-347. Huang, T.-C.; Li, C.-F.; Hwang, L.-S.; Chang, W.-H.; Lee, T-C. Bull. Nat'l. Pingtung Inst. Agric. 1982, 23, 63-66. Somers, T. C. Nature, 1966, 209, 368-370. Oszmianski, J.; Lee, C. Y. J. Agric. Food Chem. 1990, 38, 1202-1204. Huang, T.-C. In The Role of Chemistry in the Quality of Processed Foods, Fennema,O.;Chang, W.-H.; Lii, C., Eds.; AVI Publishing Co.: Westport, CT, 1986, pp. 108-117. Creasy, L. L.; Swain, T. Nature 1965, 208, 151-153. Delcour, J. Α.; Ferreira, D.; Roux,D. G. J. Chem. Soc. Perkin Trans. I. 1983, 1711-1717.

RECEIVED January 13, 1992


Changes in Phenolic Compounds During Plum ... - ACS Publications

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